Abstract

Breast cancers lacking estrogen and progesterone receptor expression and Her2 amplification exhibit distinct gene expression profiles and clinical features, and they comprise the majority of BRCA1-associated tumors. Here we demonstrated that the p53 family member p63 controls a pathway for p73-dependent cisplatin sensitivity specific to these “triple-negative” tumors. In vivo, ΔNp63 and TAp73 isoforms were coexpressed exclusively within a subset of triple-negative primary breast cancers that commonly exhibited mutational inactivation of p53. The ΔNp63α isoform promoted survival of breast cancer cells by binding TAp73 and thereby inhibiting its proapoptotic activity. Consequently, inhibition of p63 by RNA interference led to TAp73-dependent induction of proapoptotic Bcl-2 family members and apoptosis. Breast cancer cells expressing ΔNp63α and TAp73 exhibited cisplatin sensitivity that was uniquely dependent on TAp73. Thus, in response to treatment with cisplatin, but not other chemotherapeutic agents, TAp73 underwent c-Abl–dependent phosphorylation, which promoted dissociation of the ΔNp63α/TAp73 protein complex, TAp73-dependent transcription of proapoptotic Bcl-2 family members, and apoptosis. These findings define p63 as a survival factor in a subset of breast cancers; furthermore, they provide what we believe to be a novel mechanism for cisplatin sensitivity in these triple-negative cancers, and they suggest that such cancers may share the cisplatin sensitivity of BRCA1-associated tumors.

Figure 6

(A) TAp73 is tyrosine phosphorylated in response to cisplatin but not doxorubicin. Immunoprecipitated p73 was probed for anti–phosphorylated tyrosine (p-Tyr) by immunoblot 6 hours following control or cisplatin (Cis) or doxorubicin (Dox) treatment (both at IC70). The same blot was then stripped and reprobed for total p73 protein. HCC-1937 cells expressed TAp73α, while MDA-MB-468 cells expressed both TAp73α and TAp73β. (B) Induction of c-Abl–dependent TAp73 phosphorylation following cisplatin treatment. Cells were pretreated with imatinib (Ima; 1 μM for 2 hours) or vehicle control as indicated and then treated with cisplatin and analyzed as in A.